Fibrosis is a condition relating to an overproduction of collagen e.g. in the internal organs, including the kidneys, heart, lungs, stomach and joints.
Lung fibrosis is one of the predominant fibrotic diseases. Idiopathic Pulmonary Fibrosis (IPF) is characterized by chronic inflammation of the alveolar walls with progressive fibrosis, of unknown etiology. IPF, or cryptogenic fibrosing alveolitis, causes 50 to 60% of cases of idiopathic interstitial lung disease.
Usual interstitial pneumonia (UIP), a specific histopathologic pattern of interstitial pneumonia, is the classic pattern found on lung biopsy in IPF. At low magnification, the tissue appears heterogeneous, with alternating areas of normal lung, interstitial inflammation, fibrosis, and honeycombing. Interstitial inflammation consists of an alveolar septal infiltrate of lymphocytes, plasma cells, and histiocytes associated with hyperplasia of type II pneumocytes. The fibrotic zones are composed mainly of dense acellular collagen, although scattered foci of proliferating fibroblasts (fibroblastic foci), which are the sites of early and active disease, may also be seen, usually in an intra-alveolar location. Areas of honeycombing are composed of cystic fibrotic airspaces, frequently lined with bronchiolar epithelium and filled with mucus. Neutrophils may pool in the mucus. Smooth muscle hyperplasia often occurs in areas of fibrosis and honeycombing. The subpleural and paraseptal distribution, patchy character, and temporal heterogeneity are the most helpful features in identifying UIP.
An identical pattern of interstitial inflammation and fibrosis occurs in collagen vascular disorders (e.g., RA, SLE, progressive systemic sclerosis, mixed connective tissue disease, diabetes mellitus), pneumoconioses (e.g., asbestosis), radiation injury, and certain drug-induced lung diseases (e.g., by nitrofurantoin).
The clinical course of IPF is progressive; median survival is 4 to 6 yr after diagnosis. Prednisone is the usual treatment in case of IPF. Response to treatment is variable, but patients with earlier disease, at a more cellular stage before scarring predominates, appear more likely to improve with corticosteroid or cytotoxic therapy. Supportive and palliative treatment includes O2 in high concentrations to relieve hypoxemia and, if bacterial infection occurs, antibiotics. Lung transplantation has been successful in patients with end-stage lung disease.
Fibrosis of the lung relates to an accumulation in the liver of connective tissue resulting from an imbalance between production and degradation of the extracellular matrix and accentuated by the collapse and condensation of preexisting fibers.
Liver fibrosis is a common response to hepatocellular necrosis or injury, which may be induced by a wide variety of agents, e.g., any process disturbing hepatic homeostasis (especially inflammation, toxic injury, or altered hepatic blood flow) and infections of the liver (viral, bacterial, fungal, and parasitic). Numerous storage disorders resulting from inborn errors of metabolism are often associated with fibrosis, including lipid abnormalities (Gaucher's disease); glycogen storage diseases (especially types III, IV, VI, IX, and X); α1-antitrypsin deficiency; storage of exogenous substances, as seen in iron-overload syndromes (hemochromatosis) and copper storage diseases (Wilson's disease); accumulation of toxic metabolites (as in tyrosinemia, fructosemia, and galactosemia); and peroxisomal disorders (Zellweger syndrome). Numerous chemicals and drugs cause fibrosis, especially alcohol, methotrexate, isoniazid, oxyphenisatin, methyidopa, chlorpromazine, tolbutamide, and amiodarone. Disturbances of hepatic circulation (e.g., chronic heart failure, Budd-Chiari syndrome, veno-occlusive disease, portal vein thrombosis) and chronic obstruction to bile flow can lead to fibrosis. Lastly, congenital hepatic fibrosis is an autosomal recessive malformation.
The normal liver is made up of hepatocytes and sinusoids distributed within an extracellular matrix composed of collagen (predominantly types I, III, and IV) and noncoliagen proteins, including glycoproteins (e.g., fibronectin, laminin) and several proteoglycans (e.g., heparan sulfate, chondroitin sulfate, dermatan sulfate, hyaluronate). Fibroblasts, normally found only in the portal tracts, can produce collagen, large glycoproteins, and proteoglycans.
Other liver cells (particularly hepatocytes and fat-storing Kupffer, and endothelial cells) also can produce extracellular matrix components. Fat-storing cells, located beneath the sinusoidal endothelium in the space of Disse, are precursors of fibroblasts, capable of proliferating and producing an excess of extracellular matrix. The development of fibrosis from active deposition of collagen is a consequence of liver cell injury, particularly necrosis, and inflammatory cells. The precise factors released from these cells are not known, but one or more cytokines or products of lipid peroxidation are likely. Kupffer cells and activated macrophages produce inflammatory cytokines. New fibroblasts form around necrotic liver cells; increased collagen synthesis leads to scarring. Fibrosis may derive from active fibrogenesis and from impaired degradation of normal or altered collagen. Fat-storing cells, Kupffer cells, and endothelial cells are important in the clearance of type I collagen, several proteoglycans, and denatured collagens. Changes in these cells' activities may modify the extent of fibrosis. For the histopathologist, fibrous tissue may become more apparent from passive collapse and condensation of preexisting fibers.
Thus, increased synthesis or reduced degradation of collagen results in active deposition of excessive connective tissue, which affects hepatic function: (1) Pericellular fibrosis impairs cellular nutrition and results in hepatocellular atrophy. (2) Within the space of Disse, fibrous tissue accumulates around the sinusoids and obstructs the free passage of substances from the blood to the hepatocytes. (3) Fibrosis around hepatic venules and the portal tracts disturbs hepatic blood flow. Venous resistance across the liver increases from portal vein branches to sinusoids and finally to hepatic veins. All three routes can be involved.
The fibrous bands that link portal tracts with central veins also promote anastomotic channels: Arterial blood, bypassing the normal hepatocytes, is shunted to efferent hepatic veins, which further impairs hepatic function and can accentuate hepatocellular necrosis. The extent to which these processes are present determines the magnitude of hepatic dysfunction: e.g., in congenital hepatic fibrosis, large fibrous bands involve predominantly the portal regions but usually spare the hepatic parenchyma. Congenital hepatic fibrosis thus presents as portal hypertension with preserved hepatocellular function.
Scleroderma is a disease of the connective tissue characterized by fibrosis of the skin and internal organs, leading to organ failure and death (Black et al., 1998; Clements and Furst, 1996). Scleroderma has a spectrum of manifestations and a variety of therapeutic implications. It comprises localized scleroderma, systemic sclerosis, scleroderma-like disorders, and Sine scleroderma (Smith, 2000). Whilst localized scleroderma is a rare dermatologic disease associated with fibrosis and manifestations limited to skin, systemic sclerosis is a multisystem disease with variable risk for internal organ involvement and variation in the extent of skin disease. Systemic sclerosis can be diffuse or limited. Limited systemic sclerosis is also called CREST (calcinosis, Raynaud's esophageal dysfunction, sclerodactyly, telangiectasiae). Scleroderma-like disorders are believed to be related to industrial environment exposure. In Sine disease, there is internal organ involvement without skin changes.
The major manifestations of scleroderma and in particular of systemic sclerosis are inappropriate excessive collagen synthesis and deposition, endothelial dysfunction, spasm, collapse and obliteration by fibrosis.
Scleroderma is a rare disease with a stable incidence of approximately 19 cases per 1 million persons. The cause of scleroderma is unknown. However, the genetic predisposition is important. Abnormalities involve autoimmunity and alteration of endothelial cell and fibroblast function. Indeed, systemic sclerosis is probably the most severe of the auto-immune diseases with a reported 50% mortality within 5 years of diagnosis (Silman, 1991).
In terms of diagnosis, an important clinical parameter is skin thickening proximal to the metacarpophalangeal joints. Raynaud's phenomenon is a frequent, almost universal component of scleroderma. It is diagnosed by color changes of the skin upon cold exposure. Ischemia and skin thickening are symptoms of Raynaud's disease.
Several underlying biological processes are implicated in the initiation, severity and progression of the disease and include vascular dysfunction, endothelial cell activation and damage, leukocyte accumulation, auto-antibody production and crucially, an uncontrolled fibrotic response which may lead to death (Clements and Furst, 1996). Fibroblasts have a pivotal role in the pathogenesis of this disease. Primary fibroblasts obtained from patients with scleroderma exhibit many of the characteristic properties of the disease seen in vivo, notably increased extracellular matrix synthesis and deposition, notably of collagen and fibronectin, and altered growth factor and cytokine production such as of TGFβ and CTGF (Strehlow and Korn, 1998 and LeRoy, 1974).
There is no curative treatment of scleroderma. Innovative but high-risk therapy proposed autologous stem cell transplantation (Martini et al., 1999). In particular, there are currently no treatments for scleroderma targeting the fibrotic process (Wigley and Boling, 2000).
Identification of the genes associated with disease risk and scleroderma progression may lead to the development of effective strategies for intervention at various stages of the disease.
Osteoprotegerin (OPG) was first identified in 1997 as a novel cytokine secreted by fibroblasts (Simonet et al., 1997). Human OPG is a 401 amino acid protein containing a signal peptide of 21 amino acids that is cleaved before glutamic acid 22 giving rise to a 380 amino acid mature protein. Thus, OPG is a soluble protein. It is a member of the TNF receptor family (Morinaga et al., 1998, Yasuda et al., 1998), comprising four cysteine-rich TNFR like domains in its N-terminal portion (Simonet et al., 1997). OPG has been shown to have a role in the development of bone, and mice lacking the OPG gene had an osteoporotic phenotype and gross skeletal abnormalities (Min et al., 2000).
Osteoprotegerin, which is produced by osteoblasts and bone marrow stromal cells, lacks a transmembrane domain and acts as a secreted decoy receptor which has no direct signaling capacity. OPG acts by binding to its natural ligand osteoprotegerin ligand (OPGL), which is also known as RANKL (receptor activator of NF-kappaB ligand). The binding between OPG and OPGL binding prevents OPGL from activating its cognate receptor RANK, which is the osteoclast receptor vital for osteoclast differentiation, activation and survival.
Human OPG is member of the TNFR family and is a single-copy gene that consists of 5 exons and spans 29 kb of the genome (Simonet et al., 1997). Recombinant OPG exists in monomeric and dimeric forms of apparent molecular weights of 55 and 110 kDa, respectively (Simonet et al., 1997). Truncation of the N-terminal domain to cysteine 185 produces inactivation, presumably by disruption of the SS3 disulfide bond of the TNFR-like domain, whereas truncation of the C-terminal portion of the protein to amino acid 194 did not alter biological activity. Thus, the N-terminal TNFR-like domain of OPG is sufficient to prevent osteoclastogenesis (Simonet et al., 1997).
Overexpression of OPG in transgenic mice leads to profound osteopetrosis secondary to a near total lack of osteoclasts. Conversely, ablation of the OPG gene causes severe osteoporosis in mice. Ablation of OPGL or RANK also produces profound osteopetrosis, indicating the important physiological role of these proteins in regulating bone resorption. The secretion of OPG and OPGL from osteoblasts and stromal cells is regulated by numerous hormones and cytokines, often in a reciprocal manner. The relative levels of OPG and OPGL production are thought to ultimately dictate the extent of bone resorption. Excess OPGL increases bone resorption, whereas excess OPG inhibits resorption. Recombinant OPG blocks the effects of virtually all factors, which stimulate osteoclasts, in vitro and in vivo. OPG also inhibits bone resorption in a variety of animal disease models, including ovariectomy-induced osteoporosis, humoral hypercalcemia of malignancy, and experimental bone metastasis. Therefore, OPG might represent an effective therapeutic option for diseases associated with excessive osteoclast activity (Kostenuik and Shalhoub, 2001).
However, osteoprotegerin has not yet been suggested to be involved in fibrotic diseases.